2025NatAs...9.1366L

The warm neutral medium (WNM) was thought to be subsonically/transonically turbulent, and it lacks a network of intertwined filaments that are commonly seen in both molecular clouds and the cold neutral medium. We report H i 21-cm-line observations of a very-high-velocity (-330 km s^–1 < V _LSR < -250 km s^–1) cloud (VHVC), using the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST), with very high resolution and sensitivity. Such a VHVC is here clearly revealed to be a supersonic WNM system consisting of a network of velocity-coherent H i filaments. The filaments are in the forms of slim curves, hubs and webs, distributed in different layers within the position-position-velocity (ppv) data cube. The entire cloud has skewed lognormal probability distribution of column density and the filaments themselves show asymmetrical radial density profiles, indicating shock compression by supersonic magnetohydrodynamic (MHD) turbulence, as is also confirmed by our MHD simulation (sonic Mach number M _s = 3 and Alfvén Mach number M _A = 1). This suggests that hierarchical filaments can be established by shocks in a low-density WNM, where gravity is negligible, offering a viable pathway to structure formation in the earliest evolutionary phases of the interstellar medium.